Testing code which calls native methods

I have a class like this:

public final class Foo
{
public native int getBar();

public String toString()
{
return "Bar: " + getBar();
}
}

Please note that getBar() is implemented with JNI and that the class is final. I want to write a junit test to test the toString() method. For this I need to mock the getBar() method and then run the original toString() method to check the output.

My first thought was that this must be impossible but then I found PowerMock which supports testing final classes and native methods according to the feature list. But so far I had no success with it. The best thing I managed was mocking the complete class but then the test tested the mocked toString() method instead of the real one which doesn't make much sense.

So how can I use PowerMock to test this toString() method from above? I prefer using PowerMock with Mockito but if this is not possible I have no problem with using EasyMock instead.